A return to model building and flying. Part 7

The Wing

September 2004

It always surprises me how my mind can get stuck in a groov and ignore the
obvious. After struggling for quite some time to enlarge the wing plan and
print it out in A4 size sheets using a graphics program, and never quite
getting it right, I had one of those "Duuh!" moments. Suddenly it dawned on me
that I have a CAD program sitting in my system that is designed specifically
for such a task. After 15mins with Qcad, and that inluded time taking the
measurements and double checking them, the job was done and I was sticking A4
sheets together.

Then came the inexplicable problem of the size not being right. I'm sure
there is some mischievous imp that keeps altering the chord of the wing.
Inspite of carfully double checking the dimensions, every time I had the whole
thing taped together and measured it just to be sure, it was always a few
millimetres short on the chord. And of course, I had not bothered saving the
cad file as I thought it was just a temporary job! After redrawing it all
again, mercifully a quick simple job in Qcad, It took three altered printouts
before everything finally matched up.

As all the ribs are the same
size, I decided to make a cardboard template and cut them all out inividually.
Not too arduous a tast. The drawing pins help stop too much movement between
the template and the wood while cutting. Every time a make a temporary template
like this, one side is alwas different to the other. By rotating every other
one, and sandwiching them in a block, the final sanding can eliminates most of
the differences.

At this point I was beginning to get a sense of the divergence between old
and new technology. This model was designed in a time when tissue and dope was
the currency of the day. The wing relies on the tensile strengh of the covering
to give it most of it's rigity; uncoverd it is quite easy to twist and bend,
resembling a freeflight wing in a lot of ways.

My looming problem is that I will have to cover it in plastic film; not
through choice, but practical necessity. It would be very unpleasant, and it
goes without saying, unhealthy, to use cellulose dope in the cramped confines
of my flat. My experiences, admittedly not extensive, with plastic film have
not been good. I actually did some tests on dummy wing structures coverd in
lightwight and heavywight tissue, compared to film. I was quite dismayed to
find the test pieces deflected about three times as much with a given load as
the tissue and dope samples. The film was also considerably heavier. This led
to the conclusion that the airframe must be built to in a manner that regains
some of the rigity and stiffness that is lost if film is used as a covering
material. This in turn leads to an increase in weight in the structure plus the
increase in weight of the covering marerial itself. It's a bit of a vicious
circle, so I have avoided it in the past.

However, now that my back is against the wall, I do not have much choice, I
have to learn to adopt my building methods to suit. One easy stop gap measure
was to retro fit webs between the main spars this I know from experience will
give a very rigid wing for up and down defections, and to a more limited extent
in the twisting deflections in the wing. I hope this will be enough as the
alternative is to completely redesign the wing structure, which I could do
quite easily, but the main aim is to get a flying model out of this process,
not to get bogged down in the details.

Of course one should
watch what one is doing when sanding down the spar webbing, and not let the
sandng block take chunks out of the wing ribs, thus having to patch up the
holes. The results can be seen opposite.

As I have mentioned in the past, this model was designed in a way that make
it difficult to align things relative to each other. Fitting the wing to the
fuselage accurately meant removing wood from the fuselage/wing joint to give
zero incidence. The only reference I have, is the top join line of the fuselage
sides., and this was aligned with the engine bearers by eye; so I hope the
whole thing is resonably zero zero.

Because of the hybrid way of wing mounting the undercariage, I have to sheet
the top wing centre section, align everything with the fuselage, fit the under
carriage, finish sheeting the underside of the center section, all in the at
order. This model was not designed for easy construction.

Flying to date has consisted of throwing one of Pete Catlow's old PAW 2.5
powered Warlords (combat wing) about, in a manner that could be best described
as, "almost in control." Mufflers rob the engine of power which can make things
a bit hairy at times, but great fun none the less.

Trying to pilot Bob Stanley's
West Eurotech T1 36 ex-helicopter engine powered goodyear, was hair raising and
head spinning. I was really dizzy after that one, but at least capable of
standing up at the end of the flight.

I got nearly as dizzy trying to video one of Pete Moore's sedate flights
from the centre of the circle, It's very difficult squinting through a
viewfinder, walking round the pilot, and trying to keep some sense of where
every thing is. I would like to do more videoing now I can borrow a camera. At
present processig the files to a size that would be practical to place on this
website is proving a little elusive, but I am working on it.